1. Field of the Invention
The present invention relates to a portal-type sampling system for sampling the air around human beings for purposes of detecting trace chemicals present therein.
2. Description of the Related Art
The rise in worldwide terrorism, especially directed at commercial air transportation, has made it imperative that airport passenger security stations screen for concealed explosives as well as for metallic weapons. Experience has shown that concealed explosive devices have been carried onboard aircraft by terrorists on a number of occasions, some of which have resulted in disasters claiming the lives of all persons onboard. Further, the modern terrorist is sophisticated enough to obtain and use plastic explosives, a small amount of which may be sufficient to bring down an aircraft, and which are very difficult to detect.
It is well-known that specially-trained dogs can detect such concealed explosives under the proper circumstances, despite the fact that the concentration of explosive in the air may be as little as a few parts per trillion. Chemical detection devices of exquisite sensitivity have also been developed, based on the principles, for example, of mass spectrometry, ion-mobility spectrometry, or gas chromatography. Very effective devices are shown, for example, in U.S. Pat. No. 5,200,614 which issued to Anthony Jenkins and in U.S. Pat. No. 5,491,337 which issued to Anthony Jenkins and William J. McGann. Commercialized detectors that incorporate the technology of U.S. Pat. No. 5,200,614 and U.S. Pat. No. 5,491,337 typically function by initially rubbing a wipe over an article, such as a piece of luggage, that is likely to carry a trace amount of a composition of interest. The wipe then is placed in an apparatus employing the technology of U.S. Pat. No. 5,200,614 or U.S. Pat. No. 5,491,337, and an air stream is directed through the wipe to transport trace amounts of molecules of interest into the apparatus for detection. A wipe cannot realistically be rubbed across the body of a passenger to test for compositions of interest. Therefore, what has been lacking in the prior art is a rapid, convenient, socially-acceptable means for such sensors to sample the intimate environment of human subjects to screen for concealed explosives.
A hand-held sensor attached to one of the detection devices mentioned above has been used in the prior art to carry out a body-scan of an individual. Such a device is marketed by Ion Track Instruments, Inc. of Wilmington, Mass. under the trademark "VAPOR TRACER". This type of device can be used effectively at vehicle border crossings for detecting the presence of certain explosives or narcotics. However, this prior art device would be very time-consuming when applied to the many thousands of airline passengers who travel each day, and would be perceived as an intrusive approach which would be likely to elicit objections if used on a significant proportion of those passengers boarding an airplane.
Less intrusive means of screening passengers for concealed explosives have been proposed. One such prior art device is disclosed in U.S. Pat. No. 4,045,997 which issued to Showalter.
The Showalter patent discloses a horizontal "air curtain" passing between two cabinets, through which curtain human subjects pass. The air curtain is intended to "strip off" trace vapors of concealed explosive. The vapors were intended to be detected by certain sensors mounted in the receiving cabinet of the air curtain.
U.S. Pat. No. 4,202,200 issued to Ellson discloses a 10-foot-long, 7-foot-high, 3-foot-wide corridor including means to produce a horizontal circulation of air leading to a recirculation zone in the center of the device. Subjects walking through this portal are impinged upon by the recirculating airstream. As a result, it is intended by Ellson that "explosive vapor is stripped from the person by the airstream." It was further asserted by Ellson that the circulatory nature of the airflow does not further dilute the explosive vapor.
U.S. Pat. No. 4,896,547 issued to Achter et al. and discloses a "walk-in, walk-out" booth containing suction vents which horizontally draw in a "large volume" of air from around a human subject who enters the booth. Further, arrays of air-jet "puffers" and infrared strip heaters in the booth serve to "dislodge (explosive) vapors, expel air from beneath clothing, scrub vapors from exposed skin, and disrupt stagnant boundary layers of air near the person." The Ellson patent is intended to collect a non-representative sample (e.g. of less than the entire human body). A key of the Achter et al. patent is that it samples from "essentially the entire body." This patent also defines "vapor" to mean any of gas-phase, aerosol, or small-solid-particle explosive residues. The Ellson patent also asserts that a traditional metal detector may be integrated into the explosive-detection booth.
U.S. Pat. No. 4,964,309 issued to Jenkins et al. and reviewed some of the above prior art and concluded that explosive detectors of the air-curtain-type dilute explosive vapors with excess air and reduce their concentrations by as much as 100,000-fold. The Jenkins et al. patent further noted that problem with the prior art cannot be solved simply by reducing the airflow rates of such devices, since this measure would reduce the "leaching" effect of airflow on the human subject, increase losses due to natural room-air currents, and increase the sampling time unacceptably. The Jenkins et al. patent therefore proposed a different type of portal in which subjects must pass through swinging "saloon-doors" which make physical contact with the body, and which are fitted with suction pipes to remove a small sample of air from the intimate vicinity of the body. It was asserted that the physical contact of subjects with these doors removes explosive vapors from clothing and samples said vapors while not diluting them with excess volumes of air. A sampling flow rate of 0.05 to 0.2 liters/sec (0.1 to 0.4 cfm) of air was enabled due to the direct physical contact, as compared to 200 liters/sec (400 cfm) or more in air-curtain-type prior art.
U.S. Pat. No. 4,987,767 and U.S. Pat. No. 5,585,575 issued to Corrigan et al. and returned to the concept of a walk-through portal involving no physical contact with subjects, but rather an array of airjets in a recirculating air-curtain-type setup. The configuration of these airjets is claimed to "effectively isolate the internal air volume from the ambient environment." Airjets from the periphery of the portal are aimed toward the center, through which the human subject walks. The airstream is collected at the ceiling of the portal and recirculated to the airjets, after a "small amount" is drawn off for purposes of explosive vapor detection. As in the prior art, a 6-foot-long, 7-foot-high, 3-foot-wide corridor is provided for the passage of persons being screened, each person spending 2-3 seconds inside the portal. Some 2400 liters/sec (4800 cfm) of air are recirculated in total, passing through an array of airjets which produce exit airspeeds of 17 m/sec. It is specifically claimed in this patent that the directions of these airjets are critical to the functioning of the portal.
The above-described prior art perceived a need to strip, scrub, or otherwise dislodge explosive vapors and/or particles from the skin and clothing of human subjects. These vapors and/or particles are presumed to be stagnant and to require active disruption and removal in order to provide a sufficient signal to an explosives-detection analyzer.
The "sampling" action of the prior art, i.e. removing the explosive signal from the human subject and presenting it to an appropriate detector, is accomplished by a variety of intrusive means including strong air currents, continuous or intermittent impinging air jets, infrared heaters, and physical contact by swinging doors or similar devices.
Air currents used in the above-described prior art for purposes of dislodging particles from human subjects are generally oriented horizontally with respect to the vertical orientation of a standing human subject. Only in the Corrigan et al. patent is the air-current orientation not entirely horizontal (a few of the airjets in this case are oriented upward at a 45-degree angle to the horizontal).
The above-described prior art that avoids physical contact relies instead upon the movement of very large quantities of air compared to the thin layer of air surrounding the human body (to be discussed below). For those examples of the prior art that avoid physical contact, the minimum airflow is about 200 liters/sec (400 cfm) in the case of the Showalter patent, while the maximum airflow is 2400 liters/sec (4800 cfm) in the case of the Corrigan patent. This leads to a very great dilution of the chemical traces released by a subject with concealed explosives. Given such dilution, the task of detecting a vanishingly low concentration of explosive or other chemical trace in a large mass of air becomes essentially an impossible one.
The above-described prior art devices generally sample only a small portion of the airstream they create. Since available explosive analyzers can accept only a very small sample size, typically much less than 1 liter/sec, most of the generated airflow is not examined at all for the presence of trace explosives. Solid particulates are not specifically sampled or, if they are, they are subsequently boiled off to present a gaseous sample to the chemical analyzer. This heating must be done carefully to avoid decomposing the very compounds one is looking for. In the case of the Corrigan patent, a "small amount" of the recirculated airstream is drawn off for purposes of explosive vapor detection and a multi-step concentration scheme then is used in an attempt to present a higher concentration of possible trace explosives to a chemical analyzer than would otherwise be available.
The prior art recognizes that some combination of explosive vapor and/or particulates is or may be involved in the proper functioning of an explosive-detection portal. It is further asserted that such portals have broader applications, i.e. in drug and hazardous-materials detection as well. Finally, it is noted that the functions of explosive detection and metal detection, as for concealed weapons, may be integrated into a single portal-type device.